This invention relates to a snapshot of data stored in a storage system and a method for restoring a snapshot.
Frequently used as a storage system for storing corporate data is a network attached storage (NAS) connected to a local area network (LAN) to inexpensively and easily manage files. Main services provided by the NAS include file sharing services and snapshot services. The file sharing services make a NAS file system open to a plurality of NAS clients connected to the LAN to enable creation and reading of files. The snapshot services provide a snapshot which is a still image obtained at a certain time of a file system which is being run (running file system) to NAS clients. When files of the running file system are lost or destroyed by human mistakes, application faults, or the like, by rewriting a file left in the snapshot in the running file system, it is possible to restore data at the time of creating the snapshot.
Recently, the NAS has been used in many corporate backbone systems or large-scale web systems. In such a system, data availability is important. Thus, there has been an increase in demand for restoring the file system run up to then with a consistent data set as early as possible when data of the running file system is lost or destroyed.
To meet the demand, the NAS must be provided with a function of instantaneously restoring data of snapshots of a consistent data set in the running file system. Hereinafter, this function will be referred to as instantaneous restoration. As a technology of realizing instantaneous restoration, for example, a technology described in U.S. Pat. No. 6,771,843 is known. According to this technology, a writable snapshot with a file system being run (running file system) set as a master is instantaneously created. Even when data of the running file system is destroyed, by remounting the writable snapshot in the running file system, it is possible to instantaneously resume running with consistent data at the time of creating the snapshot. Additionally, by using split processing for physically splitting a master volume and a snapshot volume from each other as described in “Outline of Flex Clone Volume” (P. 4 to 6, retrieved online on Sep. 17, 2005, Internet <http://www-jp.netapp.com/tec_library/ftp/3347.pdf>), it is possible to grade up the snapshot volume to the same original primary volume. The instantaneous restoration is realized by these two functions.
Next, description will be made on a procedure necessary for an administrator to realize instantaneous restoration by using the technologies described in U.S. Pat. No. 6,771,843 and “Outline of Flex Clone Volume”. First, during NAS running, as in the case of normal read-only snapshots, the administrator periodically creates snapshots which are copies of primary volumes storing data of the running file system. Then, when the data of the primary volume is destroyed due to a certain fault, a snapshot having a data set to be restored is selected, and the running file system is mounted again in the snapshot volume.
Thus, the running file system seems to be instantaneously restored by the past snapshot. Subsequently, the administrator explicitly starts split processing. In the split processing, data in a master volume shared by the master volume and the snapshot is copied to the snapshot volume. After completion of this processing, the snapshot volume having the running file system mounted thereon can be used in the same state as that of the primary volume before the fault generation.